Variable resistors



May 7, 1963 M. E. BouRNs ETAL. 3,089,110

VARIABLE RESISTORS Filed Feb. 18. 1950 zii BY MM 3,089,l l Patented May 7, 1963 3,089,110 VARIABLE RESISTORS Marlan E. Bourne, Riverside, Calif., and Richard E.

Caddock, Ames, Iowa; said Richard E. Caddock assignor to Bourns, Inc., a corporation of California Filed Feb. 18, 1960, Ser. No. 9,535 6 Claims. (Cl. 3138-183) This invention relates to variable resistors, and, more particularly, to miniature potentiometers designed for operation under conditions of high temperature and high humidity.

This application is a continuation-in-part of United States patent application by these inventors, Serial No. 678,395, filed August l5, 1957, now Patent No. 2,938,296, for Variable Resistors.

Miniature potentiometers of the type referred to are extensively used in the electronic equipment of supersonic aircraft and guided missiles, wherein the ambient temperatures are frequently quite high due to air friction and to close proximity of extremely hot combustion chambers of rocket or jet engines. At the same time, there is need for operating the sai-d potentiometers at high power levels, which causes the resistance element to heat up due to 12R losses. These conditions of high ambient temperature and heating up of the element under high power operation, coupled with the fact that the instrument must be humidity sealed for operation in tropical climates, tend to create a high temperature condition that is conducive to rapid failure of the unit.

In the case of wire wound potentiometers, the mandrei, or card" upon which the resistance wire is wound usually has a higher coeilicient of expansion than the wire, and therefore tends to expand at a faster rate than the wire. The result is that the wire is stretched slightly at high temperature, causing a considerable change in the total resistance of the element from the `low temperature resistance value. Furthermore, as the wire windings heat up, the temperature coeflcient of the wire causes its resistance to increase still further, thereby compounding the diculty. At the same time, any parts made of organic material, such as instrument bodies of thermosetting plastic or the like, tend to decompose at `high temperatures, with deleterious effects on the electrical performance of the instrument.

One of the primary objects of the present invention, therefore, is to provide a humidity sealed variable resistor or potentiometer designed particularly for operation under high temperature conditions. This object has been achieved by providing a variable resistor made almost en tirely of inorganic material such as metal and ceramics, wherein provision is made for rapidly conducting heat away from the resistance element to the outer case of the instrument, to be dissipated to the atmosphere. In the preferred form of the invention, a metal rod, sometimes referred to as a heat sink rod, is embedded in the mandrel just below the resistance windings, and the ends of the metal rod are held by a metal support member which is in heat-conductive contact with the metal outer case of the instrument. Thus, there is an etective heat-conductive path provided from the resistance windings to the metal outer case. which acts to conduct heat away from the windings as fast as it is generated by the current carried Aby the resistance element.

Another object of the invention is to provide a variable resistor of the class described which may be hermetically sealed to make a permanently humidity proof instrument. This is achieved by constructing the outer case and inner support member of metal, so that all exposed joints may be soldered closed to make them vapor tight; while the terminal wires are brought through an outer wall of the housing `by means of glass feedthroughs, which are herineticaily sealed to the wires and to the housing. The instrument is made entirely, or substantially entirely, of inorganic material so as to withstand the heat of soldering, as well as to conduct heat rapidly from the resistance clement to the outer case.

A further object of the invention is to provide a new and improved means for terminating the windings at the end of the resistance element and also provide a length of shorted-out windings at each end of the element, for overtravel of the sliding contact. This is accomplished by providing a pair of round metal plugs which are embedded in the mandrel or card of the resistance element at opposite ends thereof; the outer surface of the plugs being iiush with the surface of the mandrel, and the wire windings being wrapped around and subsequently soldered or welded to the plugs. This arrangement has the advantage of providing a simple and inexpensive method of manufacturing resistance elements to close tolerances on total resistance, while the shorted out windings allow for a limited amount of overtravel of the contact without creating an open circuit.

Still another object of the invention is to provide a new and improved contact member for a variable resistor of the type referred to, which has a number of advantages over prior contacts. One advantage of this new contact is that it is formed entirely of sheet metal, and therefore is adapted to conduct beat rapidly from the resistance windings to the leadscrew. Another feature of the contact is that it engages the threads of the leadscrew with a resilient, yielding engagement which permits the Contact to slip with respect to the threads if the leadscrew is forced after the contact has reached the end of its travel.

Another object ofthe invention is to provide a new and improved variable resistor which is simple and inexpensive to manufacture, extremely compact in arrangement, and ruggedly constructed to withstand extreme vibration acceleration, and other external stresses to which such instruments are subjected in aircraft and missiles. A related object is to provide a wire wound resistance element of space saving, non-round configuration, but without extended llat sides that tend to cause loose wires, which permits use of a maximum length of resistance wire within a housing of rectangular cross section.

The foregoing and other obects and advantages of the invention will become apparent to those skilled in the art upon consideration of the following detailed description of one preferred embodiment thereof, reference being had to the accompanying drawings, wherein:

FIGURE l is a perspective view of a potentiometer embodying the principles of the invention;

FIGURE 2 is a cross-sectional view taken at line 2-2 of FIGURE l;

FIGURE 3 is a cross-sectional view taken at line 3 3 of FIGURE 2;

FIGURE 4 is a cross-sectional view taken at line 4 4 of FIGURE 2;

FIGURE 5 is an exploded View showing the various parts employed in the potentiometer shown in the preceding figures;

FIGURE 6 is an enlarged perspective view of the contact member employed in this potentiometer.

From a consideration of the drawings, it will be realized that FIGURE 2 also corresponds to a sectional view taken in direction of line 2 2 of FIGURE 4 and that FIGURE 3 corresponds to a sectional View taken in the direction of line 3 3 of FIGURE 4. The accompanying drawing is not to be taken as limiting this invention in any respect. Obviously a number of changes of essentially an engineering nature may be made in constructing differently appearing variable resistors or potentiometers in accordance with the basic teachings of this invention. All such changes or alterations are to be considered as part of the present invention insofar as they are defined by the appended claims.

As an aid to understanding the invention, it may be stated that it concerns variable resistors or potentiometers employing what may be termed heat sink means to maintain the temperature of resistance elements utilized in such instruments. Further, with such heat sink means it is considered necessary to employ a housing or casing which is capable of being used in maintaining the temperature Within an electrical instrument of the type indicated. Also, it is considered preferable to use specifically formed contact means as hereinafter specified so as to prevent the build up of heat within such instruments. Because of the nature of this invention, it is considered necessary to refer directly to the accompanying drawing in order to explain it more fully.

In the drawing there is shown a complete variable resistor or potentiometer 10 of the present invention which is built to include a tubular type of metal casing or housing 12. As will be apparent from the drawing this casing 12 is of a generally rectangular cross section and serves to carry a closely fitting internal support 14. Preferably this support 14 is made out of a single piece of metal and is bent so as to include a bottom 16, a front end wall 18 and a rear end wall 20; a front support wall 22 is formed so as to extend from and parallel to front end wall 18 and a rear support Wall 24 is similarly formed so as to extend from, and parallel to, the rear end wall 20, Also, it is normally preferred for structural purposes to provide various flanges 26 upon the support 14.

This support 14 is normally employed so as to hold all of the principal parts of the potentiometer 10 during the assembly of this instrument. Preferably, in the final instrument it is secured as by soldering or equivalent processes to the casing 12, although if desired, it may be merely formed so as to fit against this casing l2 in such a manner as to be held in place by means of hollow rivets 28 which fit between the front end wall 18 and the front support wall 22 and which fit between the rear end wall 20 and the rear support wall 24. Such rivets 28 may be conveniently used in mounting the entire potentiometer 10 for use.

As can best be seen in FIG. of the drawing the rear end wall 20 and the front support wall 22 and the rear support wall 24 all contain a series of aligned openings 30. The openings in the rear end wall 20 are adapted to carry small insulating bushings 32 formed out of ceramic or the equivalent. Preferably such bushings are secured to the rear end wall 20 as by solder or other equivalent techniques. Preferably these bushings 32 carry respective attached elongate Wires 34, 36 and 38, which, as indicated in FIGS. 2, 3, and 5 of the drawing, are of such dimension as to extend across the bottom of the support 14 through the openings 30 in the front support wall 22. The wires 34, 36 and 38 also all fit closely within small openings 40 in front and rear support plates 42 and 44 formed out of non-conductive material, such as for example, a ceramic. These plates 42 and 44 t closely against the front and rear support walls 22 and 24, respectively, and are held in place by certain of the anges 26 previously mentioned.

The wire 34 in the potentiometer 10 serves as an electrical return for this entire instrument and is resiliently engaged by means of a generally U-shaped end 46 of a metal contact member 48. As is best seen in FIG. 6 of the drawing this contact member 48 also includes a partially cylindrical end 50 the interior of which may be `threaded so as to resiliently engage the threads upon a threaded portion 52 of a shaft S4. lf desired, the end 50 may be considered to have essentially a U-shape. The shaft 54 includes a smooth end 56 which rotatably fits within an opening 40 in the support plate 44; it also includes another end S8 and a flange 60 which are designed to fit within and engage, respectively, a small sleeve 62. As noted in FIG. 2 of the drawing, this sleeve 62 extends through a notch 64 in the front support plate 42 and through an opening 30 in the front support wall 22. As indicated more clearly in FIG. 3, the sleeve is engaged by an end 66 projecting from a shoulder 68 on an adjusting shaft 70.

As is seen in FIGS. 2 and 3 of the drawing this adjusting shaft is carried within a generally cylindrical bearing 72 attached to the front end wall 18 and which bearing includes an external cylindrical flange 74 serving to hold a resilient, elastomeric seal 76 against the back of a slotted head 78 so as to effectively seal the potentiometer 10. Obviously the head 78 is a part of the adjusting shaft 70 and is used in order to adjust the potentiometer lll. Movement of the adjusting shaft 70 away from the remainder of the potentiometer is prevented by means of a snap ring 80l fitting within a groove 82 in this adjusting shaft 70 in such a manner that the snap ring bears against an end of the bearing 72.

As the slotted head is turned as through the use of a screwdriver or the like, the entire shaft 54 is caused to rotate. This, of course, causes the contact member to move within the portion of the potentiometer 10 between the front and rear support walls 22 and 24. As such motion occurs the ends 46 and 50 upon the contact member 48 resiliently engage both the wire 34 and the threaded portion 52 of the shaft 54. The ends 46 and 50 of the contact member 48 are capable of springing apart from the wire 34 and the threaded portion 52 of the shaft in the event this contact member 48 is moved into engagement with either of the support plates 42 or 44. This is considered to be important commercially inasmuch as it prevents damage to the entire potentiometer 10 if this eventuality occurs.

As the contact member 48 is moved as indicated, a small resilient metal arm 84 formed on this Contact member resiliently engages the wire 34 so as to effectively establish electrical contact with it. Another similar arm 86 formed on the contact member 48 also engages a resistance winding 88 located upon a non-conductive support 90. This support may be formed out of a wide variety of materials, such as any of a number of well known ceramic compositions. Preferably within the potentiometer 10 the support 90 has essentially the external shape of a triangular prism, the faces of which are joined by curved edges substantially as shown. A support having a shape of this type makes it possible to obtain effective space utilization within this potentiometer 10 and, further, permits the use of more resistance material in the resistance winding 88 than is possible with many prior types of resistance element support structures.

Within the interior of the support 90 there are formed holes 92 and 94 which extend between the ends of the support. Adjacent to each of the ends of the support 90 there is formed a cavity 96; these cavities 96 are adjacent to the holes 92. It is noted that these cavities are of a generally cylindrical configuration and that a part of each of these cavities 96 is contiguous with the exterior configuration or surfaces of the support 90. As indicated in FIGS. 2 and 4 of the drawing the wires 36 and 38 project through respective ones of the inserts 98 and respective ones of the holes 94 and 92; each of these wires is attached to one of the inserts 98 as by welding or equivalent techniques. The exposed ends of the wires 36 and 38 are supported within the openings 40 in the support plate 42.

The electrically conductive inserts 98 are arranged in such a manner that the end portions of resistance winding 88 overlay respective inserts 98 so as to provide effective termination for this winding. Preferably, the resistance winding 88 is attached as by soldering or the likc to these inserts although this is not necessary and satisfactory termination can, for many purposes, be made by merely winding a resistance wire around the inserts 98, and the support 90. It is noted that the inserts 98 are located so as to be generally between different faces or sides of this support 90 in such a manner that the wires 36 and 38 can serve to aid in supporting the support 90 and the winding 83. It may be stated that the support 90 and this winding 88 constitute a complete resistance element.

In order to achieve satisfactory heat control within the potentiometer another hole 100 similar to the holes 92 and 94 is provided within the support 90. A metal rod 102 fits closely within this hole and projects through two aligned openings 40 in the support plates 42 and 44 so as to fit closely within and against comparatively small openings 104 in the front and rear support walls 22 and 24. If desired, the ends of this rod 102 may be attached to these support walls as by (welding, soldering or other equivalent. In use the rod 102 will convey heat generated within the resistance winding 88 so as to tend to keep the temperature of this resistance winding within a usable range so that the total resistance of the winding does not change to any appreciable extent. Thus, if desired, the rod 102 may be termed a heat sink rod.

By virtue of the construction shown heat is conveyed by this rod to the front and rear support walls 22 and 24 and to the support 14, thence to the casing or housing 12. Since all of these walls and parts are formed of metal, they conduct heat readily and virtually the entire external surface of the potentiometer 10 is thus available to dissipate heat by either convection or conduction, depending upon where and how this potentiometer 10 is mounted. This is important in that the entire housing for this potentiometer 10 serves to aid in maintaining the temperature of this instrument.

Further, it will be noted that the entire potentiometer 10 is constructed in such a manner that no massive parts, such as a comparatively large plastic movable member forming a part of a contact element are employed which would tend to `build up or hold heat within the interior of this instrument. The contact member 48 is preferably formed out of comparatively thin resilient metal, and hence the control member 48 does not hold heat to any appreciable extent. Further, this will tend to conduct heat to both the shaft 54 and to the wire 34 serving as an electrical return in such a manner that these parts will tend to conduct heat away from the resistance winding 88.

Although the casing 12 and the support 14 are specifically designed in the potentiometer 10 primarily in order to obtain effective, eflicient heat control, these structures may be employed in other instruments. In a potentiometer a variable resistor with a casing such as the casing 12 and a support, such as the support 14, provide a comparatively strong instrument which is resistant to rough handling and the like which can only be bent with comparative difficulty. Further, these advantages can be obtained when these parts are formed out of other materials than are specically indicated. Further, many of the advantages of the contact member 48 make it advantageous to use this type of Contact member in other variable resistors and potentiometers.

Because of the fact that a large number ot' differently appearing potentiometers may be readily designed which utilize the essential features or principles of this invention, this invention is to be considered as being limited only by the appended claims forming a part of this disclosure.

We claim:

l. An electrical instrument comprising: a generally hollow metallic housing member; support means, comprising a metallic support member, having end wall means and positioned within said housing member in close contact with the housing member over a considerable area of the interior of the housing member to facilitate heat transfer therebetween; other means, including a resistance elem-ent and electrical return means, mounted on said wall means; means, including a shaft means, supported by said wall means and extending to the exterior of said housing; means, including a contact member resiliently engaging said resistance element, engaging said shaft means for movement thereby and electricaly connecting said electrical return means with said resistance element and constructed and arranged to be moved to move said contact means along said resistance element in response to movement of said shaft means; means including respective electrical leads connected to said resistance element and said electrical return and extending to the exterior of said housing member; and means sealing the interior of said housing member against ingress of foreign material.

2. An electrical instrument according to claim l, said resistance element comprising a ceramic mandrel and resistive means supported thereon in intimate contact therewith, and said other means comprising metallic means extending into the interior of said mandrel and disposed in close contact `with both said mandrel and said metallic support member for facilitating heat transfer between said mandrel and said metallic support member.

3. An electrical instrument comprising: a generally hollow metal housing member; means, including a metal support member having generally opposed Wall-forming portions, disposed within the housing member with close metal-to-metal contact therewith to facilitate heat-transfer therebetween; means, including a resistive means and an electrically non-conductive mandrel supporting the resistive means and having an opening extending into the interior thereof, disposed between said wall-forming portions; other means, including insulated terminal means comprising at least one terminal electrically and mechanically joined to said resistive means, extending from within to respective locations outside said housing member, said other means including means for varying the resistance interposed `between said terminal means; metal means extending into said opening and arranged in metalto-metal contact with said metal support member to form therewith a metallic heat-conducting path from within said mandrel to said metal housing member; and means serving with said support member to seal said housing member against ingress of foreign matter from the ambient exterior thereto.

4. A variable resistor comprising: a resistance element comprising essentially an elongate ceramic mandrel of generally trigo-nic cross-section with three rounded edges and said mandrel bearing an elongate resistive means along a length thereof with the resistive means exposed along at least one of said rounded edges for cooperation with a movable contact; means, including a movable contact, constructed and arranged for cooperation with said resistive means for variation of resistance exhibited between the contact and one end of the resistive means; and means, including terminal means comprising a metallic member extending longitudinally through said mandrel and serving as a support for said mandrel, said resistive means comprising a resistance element having terminations adjacent respective ends of said mandrel, and said metallic member comprising an exposed bare metallic surface forming a continuation of one of the said rounded edges of said mandrel and underlying said element at one end thereof and mechanically and electrically connected to said element.

5. A variable resistor according to claim 4, said terminal means comprising a second metallic member similar to the first mentioned metallic member and similarly disposed and underlying said resistance element at the opposite end thereof and similarly mechanically and electrically connected to said element.

6. A variable resistor according to claim 4, including metallic housing means enclosing said mandrel means and said element and said contact, and comprising means sealing said housing means and enclosed means against ingress of foreign matter detrimental thereto.

References Cited in the file of this patent UNITED STATES PATENTS 8 Wiegand May 1, 1945 Bumell Aug. 30, 1949 Podolsky et al. Jan. 10, 1956 Bourns Jan. 15, 1957 Hubbard et al Nov. 11, 1958 Bourns et al. Ian. 20, 1959 Haynman Mar. 24, 1959 Bourns et al. Apr. 14, 1959 Morrison July 14, 1959 

1. AN ELECTRICAL INSTRUMENT COMPRISING: A GENERALLY HOLLOW METALLIC HOUSING MEMBER; SUPPORT MEANS, COMPRISING A METALLIC SUPPORT MEMBER, HAVING END WALL MEANS AND POSITIONED WITHIN SAID HOUSING MEMBER IN CLOSE CONTACT WITH THE HOUSING MEMBER OVER A CONSIDERABLE AREA OF THE INTERIOR OF THE HOUSING MEMBER TO FACILITATE HEAT TRANSFER THEREBETWEEN; OTHER MEANS, INCLUDING A RESISTANCE ELEMENT AND ELECTRICAL RETURN MEANS, MOUNTED ON SAID WALL MEANS; MEANS, INCLUDING A SHAFT MEANS , SUPPORTED BY SAID WALL MEANS AND EXTENDING TO THE EXTERIOR OF SAID HOUSING; MEANS, INCLUDING A CONTACT MEMBER RESILIENTLY ENGAGING SAID RESISTANCE ELEMENT, ENGAGING SAID SHAFT MEANS FOR MOVEMENT THEREBY AND ELECTRICALLY CONNECTING SAID ELECTRICAL RETURN MEANS WITH SAID RESISTANCE ELEMENT AND CONSTRUCTED AND ARRANGED TO BE MOVED TO MOVE SAID CONTACT MEANS ALONG SAID RESISTANCE ELEMENT IN RESPONSE TO MOVEMENT OF SAID SHAFT MEANS; MEANS INCLUDING RESPECTIVE ELECTRICAL LEADS CONNECTED TO SAID RESISTANCE ELEMENT AND SAID ELECTRICAL RETURN AND EXTENDING TO THE EXTERIOR OF SAID HOUSING MEMBER; AND MEANS SEALING THE INTERIOR OF SAID HOUSING MEMBER AGAINST INGRESS OF FOREIGN MATERIAL. 